The invention relates to a method for controlling transmission power in a radio system and in a WCDMA system in particular where transmission power is controlled by means of power control steps. In this system the step size of a transmission power change is adjusted using a fixed step size or dynamically using a variable step size. An acceptable threshold value of the transmission power may vary specifically for each connection.
The WCDMA (Wideband Code Division Multiple Access) is a modulation and multiple access technique based on a well-known spread spectrum-theory, where the data transmitted by a transmitter is spread into a frequency range and identified using a code. The power of the WCDMA system must be controlled in order to maximize the operation thereof. Firstly, all powers transmitted by mobile stations should be substantially equal at the base station irrespective of multipath propagation. Secondly, only the minimum power required for reliable data transmission is allowed from the base station transmitter so that as many users as possible can share the same cell.
The WCDMA system power control can be divided into three parts. In the downlink power control, from a base station to a mobile station, the base station constantly reduces its output power until the mobile station requires for more power. The total power of the base station can thus be kept low and the capacity of one cell can be maximized without disturbing other cells.
The uplink power control, from a mobile station to a base station, is composed of an open-loop power control and a closed-loop power control. In the open-loop power control the mobile station estimates signal attenuation on a radio channel and on the basis thereof roughly adjusts its output power.
Since the radio channels in the downlink and uplink directions are at different frequencies, the open-loop estimate for signal attenuation is not necessarily accurate in the uplink direction. In the closed-loop power control the base station measures the power level of the mobile station transmission and sends a command bit to the mobile station, on the basis of which the mobile station increases or reduces the transmission power.
The closed-loop power control can be carried out using a fixed step size or dynamically using a variable step size. In the fixed step size power control the base station measures the relative power level of each mobile station signal and compares it to a threshold value. A power control command is sent to the mobile station, for example, at 1.25 ms intervals, according to which the mobile station increases or reduces the transmission power by a predetermined amount, for example 0.5 dB.
Maintaining the power control step size constant causes problems in situations where the signal or the interference level change occasionally but not constantly. If the power control in a radio system is carried out using a large fixed step size, a strong fluctuation of the power used around the desired power level becomes a problem. If the step size of the power control is small there is a risk that the power control algorithm cannot follow the rapid variations in the signal-to-interference ratio
International patent application PCT/WO97/26716 A Method for Controlling Transmitting Power and a Radio System is an example of a prior art implementation of dynamic power control. In said application the dynamic power control is carried out on the basis of several received successive power control commands in such a manner that the number of two successive diverging commands is calculated from the power control commands to be examined in proportion to the number of commands to be examined, and the calculated proportion is compared to one or more predetermined reference values and the step size is adjusted on the basis of said comparison.
Problems with the above dynamic power control system are that the transmission power of the mobile station exceeds an optimal value and that the mobile station disturbs the surrounding mobile stations and that the system does not maximize the cell capacity.
It is an object of the invention to provide a method for controlling transmission power in a WCDMA system and an apparatus implementing the method so as to achieve a balanced radio system and an improved capacity. The objects of the invention are achieved with a dynamic power control method and a WCDMA system, characterized by what is disclosed in the characterizing portions of the independent claims.
The advantage with the dynamic power control method and the WCDMA system is that power consumption is reduced in a first radio unit and therefore saves the battery of the first radio unit in case that the first radio unit is a mobile station, as the method according to the dynamic power control based on a signal-to-interference ratio (SIR) optimizes the transmission power change and the optimum power level of the transmission power is achieved irrespective of the network state and the subscriber terminal service class.
The advantages of the preferred embodiments of the invention are that the invention takes into account the WCDMA system and the services offered thereby and that signalling is reduced as unnecessary cycles are removed. In addition, the method of the invention rapidly responds to fast radio network changes, is flexible, does not cause additional signalling at the air interface between the base station and the mobile station and takes multimedia transfer requirements (for example, Quality of Service QoS) into account.
In the invention, after the power transmitted by a first radio unit, for example by a mobile station, and received by a second radio unit, for example a base station, is compared to an upper limit and a lower limit of a power margin, and when the power exceeds the upper limit of the power margin or when the power goes below the lower limit of the power margin, a command correspondingly reducing or increasing power is issued to the first radio unit at a first rate. The power control command can, for instance, be a command bit or command bits. The first radio unit can also transmit power control commands to the second radio unit.
In a preferred embodiment of the invention the second radio unit also compares the measured signal strength to an optimum value and if the measured signal strength exceeds or goes below said optimum value, the second radio unit correspondingly issues a command to the first radio unit to reduce or increase the power of the transmitted signal at a second rate which is lower than the first rate.
In another preferred embodiment of the invention forthcoming and previous power control commands and previous power control steps are taken into account when calculating the amount of power increasing or reducing the transmission power of the first radio unit.
In a further preferred embodiment of the invention the power level is not changed, if the power control commands are repeated in turns as a sequence of power control commands increasing and reducing power. If a power control command that only increases power or reduces power is repeated in the power control sequence, then the power control cycle can be delayed.
In a still further preferred embodiment of the invention the amount of power increasing or reducing the transmission power of the first radio unit is calculated as a sum of the dynamic variable step and the pre-calculated fixed step.
Still according to a preferred embodiment information is obtained by comparing Admission Control (AC) and Load Control (LC) parameters, or data, on whether a change is taking place in the radio network that would affect the set value of the first radio unit power control, and when the change takes place, for example when a new high-speed connection is established, a theoretically appropriate offset can be determined for the transmission powers of existing connections, the offset enabling the transmission powers thereof to be appropriate also after initiating the operation of a new connection in the changed network. By comparing AC and LC data and by using soft handover, if the first radio unit so desires, a balanced radio system and an improved capacity are achieved.
According to the invention the acceptable upper and lower limits of the transmission power of a particular service class and the optimum power level can change specifically for each connection and service class.
The technique of the invention can also be reversely applied in such a manner that the first radio unit controls the transmission power of the second radio unit, although in order to illustrate the description only a situation where the second radio unit controls the transmission power of the first radio unit is described in more detail.